The invention relates to a susceptor of the type comprising a circular disc adapted to support a substrate in a rotary disc reactor.
In the production of certain semiconductor devices, an epitaxial layer of silicon on a substrate, such as a silicon wafer, is used as a starting material. The epitaxial layer of silicon is deposited upon the substrate in a chemical vapor-deposition (CVD) process wherein the substrate is supported on a graphite susceptor and heated to a high temperature. A volatile compound of silicon, such as silane (SiH.sub.4), is introduced and thermally decomposed, or reacted with other gases or vapors, at the surface of the substrate to yield silicon which deposits on the substrate surface.
One type of apparatus for chemically vapor-depositing a material onto surfaces of a plurality of substrates within a reaction chamber is the rotary disc reactor. In this reactor, a plurality of disc-shaped susceptors for supporting the substrates within the chamber are positioned so that the major surfaces thereof are substantially parallel to each other in a stack-like arrangement and separated by a distance of about 5 millimeters. A plurality of gas nozzles are connected to a source of gas and positioned within the chamber so that the flow of gas therefrom is directed respectively into the spacings between the susceptors. An rf induction coil is positioned adjacent the chamber for heating the susceptors. For more detailed information on the method and apparatus of the rotary disc reactor, see U.S. Pat. Nos. 4,062,318 and 4,082,865 issued to Ban et al. on Dec. 13, 1977 and Apr. 4, 1978, respectively, and assigned to RCA Corporation.
In order to achieve the deposition of a uniform epitaxial layer having few cosmetic defects, it is necessary that the substrates heat substantially uniformly within a certain temperature range. This requires that the susceptor discs within the reactor be heated by the rf induction coil so that a large temperature gradient does not form across the surface of the disc. Referring to the prior art drawing of FIG. 1, there is shown the relative current flow when a circular graphite disc 10 is induction heated in the rotary disc reactor chamber. A large electrical current, illustrated as dotted lines 12, flows around the circumference of the disc 10 adjacent a circular recess 14 utilized for retaining the semiconductor substrate, while relatively very little current flows toward the center of the disc 10. This results in a large temperature gradient from the circumference of the disc 10, where the temperature is highest, inward to the center of the disc 10. Such a temperature gradient is undesirable when using certain CVD reactants in that it may result in the deposition of a non-uniform epitaxial layer upon the substrate. The present invention provides a novel susceptor structure for adjusting this temperature distribution in the susceptor disc 10.